I received this as a dry 6 inch stick from Hawaii ( in a little plastic bag)
Three years ago- So I planted it -inside winter out in summer and this year it is blooming! Now, I am not sure when or how to transplant to a bigger pot? It is approx 3 ft high - I will attach picture- thanks in advance! Maureen

When you repot it, just move it into a slightly larger pot with fresh soil. Do not go into too large of a pot.

Mellul Congrats on your 1st Plumeria bloom ! Btw, it appears you have a variety known as: "Celadine".

When you repot, do as Bill suggested & only step it up a little.... use a _Plain Clay Pot (Not a glazed Ceramic) so they roots can "breathe". Also the type of soil is important to Plumerias. Use a Cactus Mix like Miracle Grow but _ADD extra Perlite or some form of grit so when you water.. it runs straight through the pot.

I'm not sure it is 'Celadine'. The leaves don't look quite right. They should have 2 lines at the leaf edges.
Plumeria (Plumeria rubra 'Celadine')

Thank you for the quick response! Do I cut it back- it appears it is starting to split into 2 - maybe 2 arms from main stalk- when do I do this?

Don't cut it back. Branching is normal after blooming and makes for a better looking plant. Almost any yellow/white bloom is called Celadine. It is difficult to know for sure if they are, without knowing the heritage. Clay pots were mentioned but the vast majority of plumeria are grown in plastic pots or in the ground. Both of which work very well. If you live in the proper climate for outside plants ground is better. Most of us are not that lucky and grow in pots.

Nice job growing in the zone you are in! Make sure to give that baby all the sun it wants!
If you cut your branches, the plant will have to make new branches before it will bloom again, since blooms appear on the tips of branches only. So you effectively would skip a year (at least) of blooms. Mine only get pruned if they get in my way.

Since my plant is so leggy, should I plant deeper ?

With many trees you can't do this, but with plumeria you can. However, since you are growing it in a pot you would essentially have to put the end of the stick all the way in the bottom, which can't be good for the root system development.
Many people tend to forget that these are TREES....